✓ In vitro experiments were performed to determine the contractile activity of human serum and cerebrospinal fluid on the canine basilar artery. The majority of contractile activity in these CSF samples, which were collected 2 to 7 days following a subarachnoid hemorrhage, was proven to be due to serotonin. Serotonin was capable of producing a prolonged contraction of the artery depending on its activity. Methylsergide reversibly blocked the artery's response to serotonin and caused a contraction of the basilar artery. Phenoxybenzamine irreversibly blocked the basilar artery's response to serotonin, serum, and CSF.